In a first known optical link, the number of channels is constant and gain flatness is achieved satisfactorily. However, in transmission networks including a plurality of nodes that are interconnected via optical links, it is desirable to be free to add carrier waves to available channels or to remove busy channels from any carrier wave, at any node, using the drop and insert principle. In which case, the number of busy channels in a link of the network varies, and, if nothing is changed in that known link, then its gain does not remain constant and flat.